You have just gone through labor and birth, and are probably sore and very tired. Your body is going through a lot of hormonal changes and you probably find that your new baby just won't go to sleep at times. At the same time, you might be working on establishing breastfeeding and could have painful breasts. Finally having your baby in arms can be overwhelming emotionally as well, because of a lack of experience and the huge responsibility.
The baby blues lasts for a week to two weeks and differs from PPD. You might be reassured to hear that about 80 percent of new moms find themselves "feeling blue", emotional, and crying for seemingly no reason, after just having had a baby. This does not mean you're depressed all the time, or that you are not in love with your new baby. The baby blues comes and goes throughout the day, and does not dominate your day. Baby blues is not unlike those mood swings you probably experienced at the beginning of your pregnancy.
Those were also caused by hormonal changes and left you crying at the smallest thing! If it gets worse and you are depressed all the time, you might be suffering from Postpartum Depression. Unlike the baby blues, this is not a mild feeling of sadness that goes away after a few days or weeks, and it usually requires treatment. If you think you may have PPD, please go and see your doctor it's nothing to be ashamed of, and you can get treatment.